Various underwater systems that allow communication whilst avoiding any physical contact are known. One of the most common is based on sonar techniques. A problem with such systems is that they cannot support high bandwidth information transfer. Such systems are also degraded by noise and interference from a number of sources. Other underwater communication systems use radio links, i.e. extreme low frequency electromagnetic signals, usually for long-range communications between a surface station and a submerged vessel. These systems typically operate in the far field using physically large electric field coupled antennas and support data rates of only a few bits per second.
To overcome some of the problems associated with radar and sonar, electromagnetic transmission has been proposed for underwater communications. This offers high bandwidth communication, but over relatively short ranges. WO01/95529 describes an underwater communications system that uses electromagnetic signal transmission. This has a transmitter and a receiver, each having a metallic aerial that is surrounded by a waterproof electrically insulating material. GB0511939.1 and U.S. 60/690,966 describe underwater communications systems that use magnetically coupled electrically insulated antennas.
EM transmission systems are immune to acoustic noise and do not suffer from multipath effects due to the rapid attenuation of the EM signals. However, the short range can be restrictive where one or more of the communicating systems are mobile.